Many cyclic polysiloxanes, including pentamethylcyclopentasiloxane (sometimes referred to hereinafter as D5H) have been commercially available for a number of years. At one time, these materials seemed promising for use in medical applications. They are known to be useful as crosslinkers in silicone coatings and encapsulating materials used in the electronic industry, as well as for other electronic applications, such as composites and adhesives. Hence, synthetic methods therefor are known in the art.
For example, Haines et al U.S. Pat. No. 5,395,956, discloses a process for the synthesis of organohydrogensiloxanes, namely cyclic organohydrogensiloxanes. A hydrolyzate intermediate is rearranged via an acidic rearrangement catalyst, such as a sulfonated divinylbenzenestyrene copolymer resin, to form a cyclic organohydrogen-siloxane having the formula of formula I.
where n=3 to 12. As noted in the patent, such polysiloxanes are known in the art, but this process for its production provides minimal loss of siloxanes due to crosslinking of the siloxanes to high molecular weight byproducts. Accordingly, cyclic polysiloxanes such as pentamethylcyclopentasiloxane are known.
It is also known to create homopolymers of cyclic polysiloxanes such as pentamethylcyclopentasiloxane. For example, commonly owned U.S. Application No. 60/329,678 discloses the polymerization of cyclosiloxane molecules having the formula shown in formula I above, where R is the same or different for each cyclosiloxane moiety and is selected from the group consisting of a hydrogen, an alkyl group, an aryl group, and a cycloalkyl group, and wherein n is an integer from 3 to 8, and wherein each cyclosiloxane moiety is bound to at least two other cyclosiloxane moieties via each of at least two Si—O—Si functionalities. Polymerization is accomplished by oxidizing each cyclosiloxane moiety with water in the presence of a catalyst to form at least two Si—OH groups from the Si—H groups present on each ring and immediately thereafter condensing the cyclic rings such that the SiOH groups on each ring react to provide Si—O—Si linkages between the cyclosiloxane moieties to form a poly(cyclosiloxane) network composition. When pentamethylcyclopentasiloxane is polymerized, the product is poly(pentamethyl-cyclopentasiloxane), which is also referred to as PD5.
Crosslinking different derivatives of cyclic polysiloxanes is also known in the art. In particular, Loo, U.S. Pat. No. 5,334,688, discloses a crosslinked polymer or crosslinkable prepolymer, which is the hydrosilation reaction product of a cyclic polysiloxane, an organosilicon compound having at least two ≡Si—H groups, and an optional third ingredient, an aromatic polyene having at least one carbon-carbon double bond. In Loo, U.S. Pat. No. 5,373,077, a divisional of the '688 patent, the aromatic polyene is a required ingredient. These crosslinked polymers, while technically polycyclosiloxanes, are limited to hydrosilation reactions between cyclic polysiloxanes having the formula
wherein R is a saturated, substituted or unsubstituted alkyl or alkoxy group or a substituted or unsubstituted aryl or aryloxy group, R1 is a substituted or unsubstituted hydrocarbon group having at least one nonaromatic carbon-carbon double bond reactive via hydrosilation, and n is 3 or 4; and cyclic polysiloxanes having at least two Si—H groups. However, these compositions are prepared only by way of a hydrosilation reaction wherein a Si—H group reacts with a vinyl or allyl group to provide the Si—O—Si linkages.
Similarly, other multiple component networks have also been prepared. For example, U.S. Pat. Nos. 5,298,589 and 5,347,028 disclose “living rubbers” made from a cyclosiloxane and a polysiloxane. The polysiloxane is selected from the group consisting of a linear polydimethylsiloxane, a polydimethylcyclosiloxane containing between about 6 and about 50 silicon-oxygen bonds, a linear or cyclic block copolymer of polydimethylsiloxane and a non-siloxane organic polymer, and a linear or cyclic random copolymer of a siloxane of the formula Si(R)(R1)O where R and R1 are different and are selected from the group consisting of hydrogen, C1-C18 hydrocarbon and C2-C18 hydroxyalkyl. The cyclosiloxane and polysiloxane are polymerized by hydrosilation reactions to form polycyclosiloxanes. The resulting polycyclosiloxanes are chains of cyclosiloxanes that are linked via crosslinking moieties that contain alkyl groups. That is, the siloxane moieties of the cyclosiloxanes are linked via —R—Si—R— or —R—Si—O—Si—R— linking groups, where R is a hydrocarbon. Alternatively, a cyclosiloxane may be contacted with chlorine gas to form a chlorosilane. The chlorosilane is then contacted with water to hydrolyze it and form a silanol or a siloxane, which is then heated to form a liquid polycyclosiloxane. The polycyclosiloxanes disclosed in these patents are essentially linear chains of cyclosiloxanes with minimal branching. This structure dictates their liquid state. These polycyclosiloxanes are then treated with a strong acid, which reportedly rearranges the Si—O bonds, to cure the composition and form thermally reversible rubbers.
Another example of multicomponent networks is provided by commonly owned U.S. application Ser. No. 09/833,774, the disclosure of which is hereby incorporated by reference herein, which discloses multicomponent networks comprising the reaction product of a plurality of multifunctional, allyl-terminated polyethylene glycols linked to a plurality of multifunctional siloxanes having at least two SiH moieties for each siloxane.
However, the need continues to exist for additional silicon-based compositions that exhibit thermal and oxidative resistance and flexibility. A need also exists for a method of making a polycyclosiloxane network that does not depend on treatment of a polycyclosiloxane with a strong acid to form the network.